Structure arrays in MATLAB

Introduction

Structure arrays let you group related data with different types in one variable. This helps organize information clearly.

When you want to store information about multiple people, like names, ages, and heights.

When you need to keep different types of data together, such as text and numbers for each item.

When you want to access data by names instead of just positions.

When you want to add new fields later without changing the whole data layout.

Syntax

MATLAB

structArray = struct('field1', {value1, value2, ...}, 'field2', {value1, value2, ...});

Each field holds data for all elements in the structure array.

Use curly braces {} to assign multiple values to fields for each element.

Examples

Creates a structure array with two elements. Each has a name and age.

MATLAB

people = struct('name', {'Alice', 'Bob'}, 'age', {25, 30});

Creates an empty structure array with fields but no elements.

MATLAB

emptyStruct = struct('name', {}, 'age', {});

Structure array with one element.

MATLAB

onePerson = struct('name', {'Charlie'}, 'age', {40});

Adds a third element to the existing structure array.

MATLAB

people(3).name = 'Diana'; people(3).age = 22;

Sample Program

This program creates a structure array with two people, shows it, adds a third person, and shows the updated array.

MATLAB

people = struct('name', {'Alice', 'Bob'}, 'age', {25, 30});
disp('Before adding new person:');
disp(people);

people(3).name = 'Charlie';
people(3).age = 40;

disp('After adding new person:');
disp(people);

Important Notes

Accessing fields is fast because data is organized by field names.

Adding elements at the end is easy, but inserting in the middle requires shifting elements.

Use structure arrays when you want to keep related but different types of data together.

Time complexity for adding an element at the end is O(1), but inserting in the middle is O(n).

Space complexity grows with the number of elements and fields.

Common mistake: Mixing cell arrays and structure arrays syntax can cause errors.

Summary

Structure arrays group related data with different types in one variable.

Use them to organize data by named fields for easy access.

You can add, access, and modify elements using field names and indices.

Practice

1. What is a structure array in MATLAB used for?

easy

A. To perform matrix multiplication

B. To store only numeric arrays

C. To group related data with different types under named fields

D. To create loops for iteration

Solution

1. Step 1: Understand the purpose of structure arrays

   Structure arrays allow grouping of different types of data under named fields.

2. Step 2: Compare options with the definition

   Only To group related data with different types under named fields correctly describes this use; others describe unrelated concepts.

3. Final Answer:

   To group related data with different types under named fields -> Option C

4. Quick Check:

   Structure arrays = group data by fields [OK]

Hint: Remember: structures group different data types by names [OK]

Common Mistakes:

• Thinking structures only hold numeric data
• Confusing structures with loops or matrices

2. Which of the following is the correct way to create a structure array with fields 'Name' and 'Age' in MATLAB?

easy

A. person = struct('Name', 'Alice', 'Age', 30);

B. person = struct(Name = 'Alice', Age = 30);

C. person = struct['Name', 'Alice', 'Age', 30];

D. person = struct{'Name', 'Alice', 'Age', 30};

Solution

1. Step 1: Recall MATLAB struct syntax

   MATLAB uses struct('field1', value1, 'field2', value2) to create structures.

2. Step 2: Check each option's syntax

   person = struct('Name', 'Alice', 'Age', 30); uses correct parentheses and quotes. Options B, C, D use invalid syntax.

3. Final Answer:

   person = struct('Name', 'Alice', 'Age', 30); -> Option A

4. Quick Check:

   struct uses parentheses and quotes [OK]

Hint: Use parentheses and quotes with struct() [OK]

Common Mistakes:

• Using square or curly brackets instead of parentheses
• Omitting quotes around field names

3. What is the output of the following MATLAB code?

student(1).Name = 'John';
student(1).Score = 85;
student(2).Name = 'Jane';
student(2).Score = 92;
disp(student(2).Name);

medium

A. Jane

B. John

C. 85

D. 92

Solution

1. Step 1: Understand structure array indexing

   student(2) accesses the second element; .Name accesses the 'Name' field.

2. Step 2: Identify the value stored in student(2).Name

   It is 'Jane' as assigned in the code.

3. Final Answer:

   Jane -> Option A

4. Quick Check:

   student(2).Name = 'Jane' [OK]

Hint: Use index and field name to get value [OK]

Common Mistakes:

• Confusing index 1 and 2
• Accessing Score instead of Name

4. Identify the error in this MATLAB code snippet:

car.Make = 'Toyota';
car(1).Model = 'Corolla';
car(1).Year = 2020;

medium

A. Field names must be numeric

B. Missing semicolon after first line

C. Mixing scalar and array structure assignment causes error

D. No error, code runs fine

Solution

1. Step 1: Analyze structure assignment styles

   First line creates scalar structure car with field Make. Next lines assign to car(1), valid for scalar structures.

2. Step 2: Understand MATLAB structure array rules

   MATLAB allows indexing scalar structures and dynamically adding fields; no error occurs.

3. Final Answer:

   No error, code runs fine -> Option D

4. Quick Check:

   Scalar struct indexing + field addition works [OK]

Hint: Scalar structures can be indexed like arrays [OK]

Common Mistakes:

• Thinking scalar and array assignments cannot mix
• Believing field names must be numeric
• Assuming missing semicolons cause errors

5. Given a structure array books with fields Title and Pages, how do you create a new structure array shortBooks containing only books with less than 100 pages?

hard

A. shortBooks = books(books.Pages < 100);

B. shortBooks = books([books.Pages] < 100);

C. shortBooks = books{books.Pages < 100};

D. shortBooks = books(books.Pages > 100);

Solution

1. Step 1: Extract numeric array of Pages

   Use [books.Pages] to get all Pages values as a numeric array.

2. Step 2: Use logical indexing to filter

   Compare with < 100 to get logical indices, then index books with these.

3. Step 3: Check syntax correctness

   shortBooks = books([books.Pages] < 100); correctly uses parentheses and logical indexing; others are incorrect.

4. Final Answer:

   shortBooks = books([books.Pages] < 100); -> Option B

5. Quick Check:

   Logical indexing with [books.Pages] works [OK]

Hint: Use [struct.field] for numeric indexing [OK]

Common Mistakes:

• Using curly braces {} instead of parentheses ()
• Forgetting to use brackets [] around field extraction
• Using wrong comparison operator